ATC here - with modern Mode-S downlinked aircraft parameters, we can see your MCP selected altitude downlinked to your aircraft coupled tag on our radar screen.
If your downlinked level is different to the cleared level that we have selected, a breakthrough warning message pops up and we will say over RT "confirm your cleared level is FL100" etc. It's a really great tool to prevent level busts. The other day I said "Climb to FL230" which was readback correctly, but FL320 was set in the MCP. Error message, inquire with crew, error fixed. Level bust prevented 10,000 feet below where it might have been an issue.
Anyway just a point to say, I don't think I've ever seen this radar warning about a selected level above your cleared level, when speaking about meeting ALT contraints. But I'm in EASA land. I can see the benefit of changing your MCP alt when approaching the waypoint to the next FMC alt, as long as it's all above your cleared level, but just a note from the ATC side that I don't often if ever see it happen. Unless your Mode-S output is deceiving us
